Display devices which are driven with transistors including amorphous silicon or polysilicon have been mainly used. However, it has been difficult to realize multi-gray scale display in these display devices due to the effect of the off-state current of the transistors.
As an example of a pixel in a display device, FIG. 12 illustrates a pixel including a transistor 100, a liquid crystal element 110, and a storage capacitor 120. The transistor 100 is an amorphous silicon transistor or a polysilicon transistor. In each pixel, image data is written into the liquid crystal element 110 and the storage capacitor 120 from the transistor 100, so that the liquid crystal element 110 is provided with an electric filed; accordingly, an image can be displayed.
However, the electric charges accumulated in the liquid crystal element 110 and the storage capacitor 120 are discharged due to the off-state current of the transistor 100, and the voltage of the pixel is accordingly fluctuated.
In each pixel, a storage capacitor C, a voltage fluctuation V, an off-state current i, and a holding time T satisfies the relation of CV=iT. Therefore, in the case where the off-state current i of the transistor 100 is set to 0.1 [pA], the electrostatic capacitance C of the storage capacitor 120 is set to 0.1 [pF], and one frame period T is set to 16.6 [ms], the voltage fluctuation V of a pixel in one frame period is 16.6 [mV] because the product of the electrostatic capacitance and the voltage fluctuation is equal to the product of the off-state current and the holding time.
It is assumed that the display device has 256 (=28) gray scales and the maximum driving voltage of the liquid crystal element of a pixel is 5 [V]. In this case, the grayscale voltage for 1 gray scale is approximately 20 [mV]. In other words, the voltage fluctuation V of the pixel=16.6 [mV] which is obtained in the above calculation corresponds to the fluctuation of the grayscale voltage for approximately 1 gray scale.
Further, it is assumed that the display device has 1024 (=210) gray scales. In this case, the grayscale voltage for 1 gray scale is approximately 5 [mV]. Therefore, the voltge fluctuation V of the pixel=16.6 [mV] corresponds to the fluctuation of approximately 4 gray scales and thus the effect of voltage fluctuation due to the off-state current cannot be ignored.
In Patent Document 1, a display device including a polysilicon transistor has been proposed.